The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
Why would Germany want the US fighting against it, if they wanted their support against the British?
You do not specify if you are referring to World War I or World War II.
If you are talking about World War I, what happened was that the United States was a natural ally of the British for historic, political, and economic reasons.
Indeed, US President Woodrow Wilson had decided to remain neutral during the first stages of the war. He supported the allies with money loans, supplies, and weaponry.
However, everything changed when the US intercepted Zimmerman's telegram in which Germany asked México for his support in the war. That and the sinking of the Lusitania ship by the German navy made President Wilson ask Congress for a declaration of war against Germany.